I think your best bet is http://www.go2pac.net/flash800.html
(Their OEM-1 interface has gain adjustments)

If you email them they should be able to give you part numbers for their adaptors. They don't sell direct but you you can buy their stuff from http://www.tune-town.com (Who can also look up the correct part numbers)

That's how I hooked up to my BOSE stereo and it works great!. I'm pretty sure you can do the same with a MACH system. Good Luck.