I thought of doing this some time back, but using copper tubing wrapped
around the exhaust manifold rather than the heater hoses. Advantage would be
that you'd have near instant hot water, even after running the engine for a
minute in the snow or whatever. Disadvantage would be that the water would
get too hot pretty fast, and the copper would need a pressure valve to let
off steam. My idea was to install another faucet in the sink, not put in a
shower. It seems like however you configure it, you could make the whole
shebang from stuff at the hardware store.