If there is one without a gain adjustment, it's easy enough to rig up a couple of potentiometers to be your gain adjustment. Audio input to middle terminal, one outside terminal to amp input, other outside terminal to shield/ground of head unit output/amp input.
Dunno what size (1K, 10K, 100K?) of potentiometers you'd want to use but they're cheap, so experimentation won't cost much...
