1988 Dodge Dakota electrical problems

Recently replaced the battery, I was told by my son that before this that the dome light went out and fuse was still good(For no apparent reason). The truck was running fine. He had a radio installed (he could not get it to work)and the tech told him that he had "some" electrical" problems, do not know what kind.
He came home from school and the truck would not turn over at all, the lights would not come on and there were a few lights on the radio that were illuminated(this without the key in the ignition). As soon as you attempted to turn the lights on, the lights on the radio went off.
The few lights(outline lights only) on the radio are all that works at this point, any suggestions?