Switched decks, horrible bass
 

Apologies if this comes off as a newbie question, I don't know much about car audio.

My boyfriend owns an '01 Celica. Two years ago, he changed the stock deck for a Sony mex-1gp. The work was done by the guys at A&B Sound. When my boyfriend picked up his car, the guy who did the installation mentioned something about the harness (he can't remember exactly what now). Now the problem is: since the deck was changed the bass is extremely poor, even with the bass settings on max. On the stock deck, there was at least some decent bass coming from the speakers. I want to help him out so I'm wondering what kind of problem this might be and how it can be fixed. Any help is much appreciated! :)

If the deck has crossovers you might want to try turning off the high pass filter

The deck doesn't have this ability, as far as I know.

There's a good chance that one or more of your speakers are out of phase. Take it to a shop for a diagnoses.

It might be a GT with a factory amplifier. Might need proper re-wiring or an out of phase speaker(s) like installerz mentioned.