The box is a (passive) crossover. He probably had components in there, so the crossover took the signal and filtered out frequencies for corresponding speakers (ex, only high freq to tweeters, mid to low for doors). You put your door speaker signal in, and it splits it out to woofer/tweeter normally.
I can go into more detail if you want. If you just want to know what the box is, there ya go. If you let me know what speakers are in there currently, I can give you some suggestions as to what I would do.