when i bought the Swift VXI, I turned in my old Wagon R that had basically a new HU and speakers, I was told at the ASS
that transferring my HU and speakers will not void my warranty as long as I didnt cut any wires. Basically, the HU has a socket prelaid in the car's wiring harness to allow the service centre to sell their own (and exorbitantly priced) accessories with the car. They simply connect the HU to the socket. If you do the same cheaply from outside, you wont have warranty issues with the car itself, though they will not touch any problems with the HU.
Similarly, when I added 90/100 bulbs to me headlights, I did the socket-to-socket approach. (I have since reverted back to the 55W xtremevision bulbs using Philips bulbs instead of stock to get better illumination.) Do go for quality branded stuff though.
But there are some things that will require you to get them installed from the ASS
. One is the reverse parking sensor system that replaces the rear view mirror. Another is power windows and central locking. Typically, these dont come with the low spec trims and require opening up a lot of panels. Outside fitters really dont care and often break the interior panels while opening them (as in my case he broke the stereo panel and it took a month to replace it from the ASS
). Best to fit these things at the ASS
Note that most manufacturers overengineer their engines and gear boxes, so these almost never go wrong. Electrical problems, though, are very common in cars
In case you have any problems with fuses during installing the aftermarket spares, most manufacturers have spare fuses in the fusebox cover itself and good electricians know where these are located.
For those who are buying a new car and reading this, going for the top spec version is usually cheaper that getting stuff installed from aftermarket. SO if you have the money, go for it