Johnny Cash covered mercy seat on the last album he did.
It is really really good.
You should pick it up if you get a chance, it's just calle 'Cash' and has a black and white silhouette photo of him back stage at a concert or something. There's also a cover of U2's 'One' and Tom Petty's 'Won't back down'. Oh, and Tom Petty and the Heartbreakers are his backup band on the whole album. Really good stuff.