Surely you could find an XM tuner that presents audio out of RCA plugs...

Bitt, there aren't many. A search of Crutchfield reveals two units - which is more than I found a few months ago when looking for an XM tuner that did exactly that. One of these units is Sirius, though, not XM.

Jensen SSR2000 Sirius Satellite Tuner and Controller
Sony DRN-XM01C2 XM Digital Satellite Radio -Cassette pkg

Everything else I could find is either compatible only with specific aftermarket head units or works through an FM modulator.