I have a BMW 318i 1993 and when i got it there was a big speaker and amp hooked up. I unhooked it but now i cant get any sound....the radio lites up but no sound. i know speakers work cuz they worked...
Without knowing exactly what modifications were made to the factory sound system, it's difficult to point you to a particular fix. The number of channels on the "big amp" that you disconnected would help. Even the make and model.
But, from your description it's possible that the previous owner installed a multichannel amp to drive both the "extra bass speaker", usually referred to as a subwoofer, and the original vehicle speakers. In that case, the speaker wiring from the head unit (receiver) was cut and the receiver side of the wires was rerouted to the input of the aftermarket amp. The output of the amp was then connected to the other ends of the speaker wiring.
To restore to the original, you need to pull the stock receiver and resplice the wiring. If the previous owner cut the plug completely, it may be necessary to purchase a replacement from your BMW dealers service or parts department. It's possible, but not likely that the previous owner used both an aftermarket "harness" and an aftermarket "reverse harness" to avoid having to cut the wiring. In that case, all you'd have to do would be remove them and plug the radio harness back into the vehicle harness (or the back of the radio). But most DIY audio enthusists don't mind butchering the vehicle wiring. And you'll probably find that electrical tape was used to secure the wiring. After a year or so under the dash, it'll be a sticky mess to unravel.
The speaker wiring colors for the '93 318i are as follows:
Left Front Speaker Wire (+): Yellow/Red
Left Front Speaker Wire (-): Brown/Orange
Right Front Speaker Wire (+): Black/Red
Right Front Speaker Wire (-): Brown/Orange
Left Rear Speaker Wire (+): Yellow/Black
Left Rear Speaker Wire (-): Black/Orange
Right Rear Speaker Wire (+): Blue/Black
Right Rear Speaker Wire (-): Brown/Orange
Hope this helps.