dvd sound too low

here's the problem

My neighbor recently purchased a used 2003 ford conversion van with a small CRT TV and a separate VHS and DVD player.  Well the wiring up in the entertainment center was a DISASTER im amazed it didn't burn the whole mess to the ground.   

after sorting out all the horrible wiring and bare wires getting everything operative and functional, NOW when you play a DVD from the Audiovox DVD player the sound output is low.  

When listening to regular TV broadcasts or even using a gaming system the sound is fine.     The gaming system is the loudest of the 3 sources.  Using the headphone jacks in the back of the van and the independant volume controls at each jack you can BLAST any of the 4 audio sources but even with the volume on the TV cranked all the way up the DVD is too quiet to be heard of regular road noise and just loud enough when parked.

All of the rear sound goes through the TV itself then out to the speakers in the van.   The audio signal from the DVD player is MONO and is split into a Y to feed the right and left audio outputs from the TV.

Can he install a cheep mono amplifier using RCA's to between the audio output on the DVD player and the Input to the TV?