help. no sounds that i like

i recently aquired a 98 328i . it had been sitting a while so the battery was dead. the radio needed a security code to operate so i entered the code found in the glove box. the previous owner clearly had an aftermarket sysytem consisting of an amp and an infiniti basslink in the trunk. the radio works but despite playing with every control on the stock factory head unit (balance/fade/ ect...) i get absolutely no noise from the speakers in the interior. in fact , the only sound is emmitted from the infiniti basslink. (mostly unintelligable gibberish) . i played with three knobs on the sub (gain ,tone ect...) and nothing changed for the inside speakers. Is there something i am missing here? it took me three days to get the heater to blow hot air due to an odd swith belkow the center vent. lol. i am hoping there is another simple fix. the stuff was pro installed as the reciept for labor only was 400 bucks. i assume perhaps speakers were added or replaced. there are two plexiglass cases mounted under the rear window deck for tiny tweeters that look aftermarket. Is there somekind of fuse or switch on either the sub, amp, or factory head that i can look for ....or play with...or stare at while i stand next to this bleating box of poop? THANKS IN ADVANCE. as you can tell...i am not an audiphile at all.

blown second amp fuse. all good now! need a magazine for an hk cd changer and it is completely operational.