Mine sits on top of the passenger foot box inside the engine compartment. (That's the driver's side to you guys).
Never had a problem in 12,000 miles, including some classic UK wet weather driving.
Mind you, I only have a tiny block, so I suppose the miniscule amount of heat it emits (couple of candles-worth tops) wouldn't affect the MSD anyway
One thing I did fit was the MSD-supplied capacitor across the feed lines, to stop any voltage spikes from affecting the MSD as I pull the battery isolator switch key. Seems to work fine.