The best thing to do is replace the radio with something more adequate for what you have planned... maybe something with mp3 capable.

If you plan on keeping the volvo deck you can always use hi-input to the amp instead of using what is preferred, low-input (rca style)

if your friend thinks he can go around the built in amp then go right ahead but most likely you'll wreck the volvo deck all together and blow alot of fuses. (remember you dont want to do anything that can wreck the computer) or else you will be plaqued with all sorts of problems. IMO