The base Stereo system is similar in most countries, it seems.
That one-speaker system has been seen in the EU as well.
The signal out should be the one described here, I bet:
So you either need to recode the HU to Hi-Fi mode, converting the HU outputs to a lower voltage and turning off the internal processing, or you need an external speaker-level-input reversing processor.
Basically, you need a processor like an Audison BitOne or a RF 360.2 or a JL CleanSweep or a JBL MS-8 - or you need a manual DSP device and an installer who's really skilled and has great test gear.
We use Zapco DC amplifiers a lot. The MS-8 probably isn't available in HK yet, but a lot of people use those, add a center channel.
If you recode it to HiFi mode, buy the missing tweeter flanges, then you're set.