Think I may have found a cheaper answer instead of adding pac-rom units.
I recently fitted two stereos, one JVC double din unit to celtic vals car and another pioneer unit to keyser's wifes car.
When fitting the unit to vals car unfortunately the head unit didnt have the necessary rca outputs. With Vals car and the necessity for her to get back to her home and with us having the dash all stripped down i decided to take the gamble and cut the red and white and yellow and black rca ends off of the iso lead set.
After cutting the ends off close to the white sockets at the end of the leads i found the necesary colour coded speaker wires. After soldering these to the high imput speaker wires on the head units plug i refitted the stereo.
I was quite shocked how loud the unit was in comparisson to my sony unit fitted using the correct iso leads ie rca's and the pac-rom units.
I didnt know if it was just the case of the jvc unit being more powerful but with val happy with what i had done we sent her on her way.
Recently i fitted a pioneer unit to Keyser's wife's zed in the same way as val's with the same results in sound quality (loudness).
So tonight i decided to strip my dash down remove my pac-rom units and carry out the same modifications to the loom as above. I am pleased to be able to report that the sound output is tenfold more than it was with the rom units. Hopefully this may be of help.