I don't often read the Maxima forums, but was looking for something else when I ran across this Aux Input Adapter. I'd go with that option, except I have the one known model that doesn't work as listed on the bottom of this page.

Also of note is this post in which he seems to have the same problem I had:

I wanted to run from the HU's RCA preamps to the Bose amp, but the volume wasn't there. I had the HU all the way maxed out at 35 and the sound wasn't all that loud. So I tried hooking up the HU's RCA preamps to the PAC kit, but I had the same problem.

I guess I should've done more research before buying stuff...