The Hi Fi harness from Technic is pretty foolproof
It doesn't leave much for you to mess up if its like the ones
that came with mine.
The crossover that came with the Jehnerts just goes between the
tweeters in the doors and the doors is this correct?
Some people have pulled out the Inputs at the amp to see if noise still
persists. If it doesn't it suggests the noise is coming from the head unit.
This would mean the Head Unit is not grounded with respect to the
Amp or defective .
Where did you put the amp?