Most HD radios are standard radios (single DIN). The HD comes from a box about 5" x 5" x 1.5" that plugs into the back, like the Sirius boxes. You just have to find a place to stash the HD box.
BTW, HD Radio does NOT stand for hi-def. It's the same fidelity as FM, they just copped HD to cash in on the hi-def hype.
Mike