As I bought the california without speakers and radio, I had no rear speakers nor front tweeters. I just had them installed (good quality) and the front speakers replaced by quality speakers as well for EUR 680. It was an 6.5 hour job. For the speaker cable, they took the route on the opposite side from the sliding door (it's a beach) because then the floor could be left untouched. Then they traversed through the back (bumper protector) for the speaker grill on the sliding door. Entire interior panels had to be removed, as well as the middle seats. If you have the heater, the sliding door side only fits a 13cm rear, otherwise it's a 16cm.
I now have a high quality 6-speaker system, with this great android system, for about 1.1K EUR. Sounds great and crisp with a lot more bass.
Next is to test the DVB-T2 receiver I got for digital radio reception.
As for 3G dongle, it worked fine through pp widget2, but I've used a HUAWEI E5330S-2 in the glove box, powered from the Newsmy, which is great and can provide internet access on its battery when the car is off. You need another sim though. Alternatively, tether your phone's data to the newsmy. The core bluetooth function of the newsmy (there are 2, 1st is for bluetooth data and headphone, 2nd is for calling) can auto connect through your phone.