hot wire under the hood

I'm not sure what you are looking for. Are you looking for a specific wire that is already there, or to add a switched circuit under the hood?

"party lights" that won't drain the battery, but are switched is probably what he's after, there is always a danger when "sharing-or piggy backing" the hot side, best find an open fuse receptacle and use that- Inline fuses are always a good idea, but consider your source, are you going to tap right into the ignition? I predict a fire happening, best not do this~

the cigarette lighter (if switched) is a good source~ not needed 'cause no one smokes IN MYCAR what are you crazy?...LOL seriously a good fused source think about it's got to heat up a piece of wire red-hot, if it were wimpy would blow in two seconds~

test light wires key on, key off under hood ,wiper ,fan, ,[fuse box tap] etc. then use it to activate a relay carrying the heavy, current,if more than 5 amps or so

It all depends on what you plan on providing the power to. If it's low power, grab power from a fuse in the under hood fuse box. If it's a heavy load, get power from the battery and use ignition power to fire a relay. The cigarette lighter is a good source if the power shuts off with the key. My GM lighter socket is hot all the time.