If you installed an aftermarket radio and then find your dome light doesn't work anymore when you open a door, its because the dome light circuit went through the original stock radio.
Its a easy fix if you DON'T have keyless entry option (thats another story).

Pull radio out and look in the cavity where it was, you should have a 16pin gray connector (radio related) , a disconnected 22 pin green connector (security system, not used) and a disconnected 16 pin green connector.

The disconnected 16 pin green connector needs a jumper installed from pin 8 to pin 14 to complete the domelight circuit and fix the issue. Refer to photos for more info but basically put a jumper between the far lower right socket and the 3rd from left socket on same row (bottom row). That should give you your, door activated, dome light back. =)