I searched the forums and found very little information on swapping sports seats into a car that didn't have power seats. If your car doesn't have power seats, there is no power wire going to the seat harness to tap into.
I just purchased a set of manual sport seats out of a 07 328i and put them in my 06 325i that does not have power seats. The seats themselves bolt in without issue, and work fine. However, the manual sport seats have power adjustable side bolsters, so if you want them to work correctly, you will need to hook up power to the actuators.
So I just reminisced back on my HS and college days when my friends and I were hooking up cheap car stereo systems in each others cars, and came up with an idea of pulling power from a circuit at the fuse box. I'm using the same circuit that the power windows are on, and it works fine.
What you'll need:
1. Sports Seats (duh)
2. Wire cutters/stripping tool
3. wire-tap splice connectors (other connectors optional)
4. 14g wire (about 20-25 feet)
5. Typical torx bits and drivers
6. Electrical tape
7. Zip ties
1. disconnect the battery. My friend with an E46 said we only needed to make sure the key wasn't in the ignition, but on the E90, you will still get a CEL for the airbags and seatbelt if you disconnect the seat harness while the battery is connected. So just remove the negative battery terminal, and tuck it out of the way.
2. I started with the driver's side, because it was furthest away from the fuse box. Remove all 4 seat bolts if you haven't already, and just tilt it back, so the bottom is exposed. Unplug the wiring harness if you haven't already and unhook it from the seat bracket, so it is easier to work with.
3. cut open the seat's wiring harness, near the plug. You are looking for a red/blue wire (12v power) and a brown wire (ground). they are thicker than most of the other wires going to the seats.
4. Once these wires are located, you want to tap into them, and connect your own wires to run to power and ground to a different location. I used wire-tap splice connectors because I didn't want to cut the factory wire. Start by installing a wire-tap splice on the power wire, along with the end of your own wire.
5. After you have tapped into the power wire on the seat, you need to get that wire over to the fuse box, located behind the glove box. The easiest way to do this, is to rout it under the dash, in front of the center console area.
5a. Remove the lower kick panel on the driver's side. There are 3 torx screws that have to come out, and then you can simply pull the panel out and disconnect the wiring to the convenience light.
5b. Remove the lower kick panel on the passenger side. There are 2 torx screws that need to come out here, located on each side of the glove box hinge.
6. Now you'll need to pull the wire from the driver's side to the passenger side. This is where a shirt hanger comes in handy. The kind you get from the dry cleaners are easier to work with. Grab on, untwist it, and straighten it out. Fold the end over like a hook, and fish it through the space you just exposed in front of the center console. I found it was easier to push it through on the passenger side.
Now hook the end of your wire to the hanger, and pull it back through to the passenger side.
7. Once you have pulled the wire through, you can work on concealing the wire. Make sure to leave enough slack so that the seat can move freely. I just "traced" it over the factory wiring harness and zip tied it in place where the factory harness comes out of the carpet.
You can tuck the wire under the carpet seam that runs under the seats and up to the center tunnel. Then tuck it under the edge of the center console. pull any extra slack out to the passenger side. Then remove the cover on the back of the glove box, exposing the fuse box, and pull the wire up through the space underneath. You can use a few more zip ties to secure the wire under the passenger side dash. Pull it out the front of the glove box, and you can cut it there (make sure to leave enough slack to work with).
8. Now tap into the brown ground wire located next to the red wire you just tapped into with a new piece of wire (you only need a couple of feet for this). run it along the factory harness to where it meets the carpet and zip tie it in place (this will give you adequate slack for seat movement).
We are going to run this ground wire to where the seat rail bolts to the floor. You can tuck it under the carpet like you did with the other wire, then out to the seat rail hole.
At this point, you want to put tape over the wire taps you just made and zip tie all the wires to the seat harness to keep them tidy.
9. Reinstall the seat, but before you put the bolt into the seat rail pull the ground wire up through it, and fold it over. Place a washer over it before you thread the bolt through, so that the wire doesn't spin or get cut as you are tightening the seat down.
10. Repeat this process for the passenger side seat with the wire you have left.
11. Locate the 30 amp window fuses in the fuse box. There are empty locations for where the seat fuses are supposed to be, but mine didn't have any wire terminals connected to them.
Pull out two of the 30 amp fuses, the slot looks like a "+" sign. I inserted the wire into the top part of the slot and then shoved the fuse back into the slot, and it held the wire tight.
When you are complete, it should look something like this.
I've been driving with the seats wired like this for the last few days, and everything works great.