Wire straight to the battery -- BOTH terminals (I'm not a fan of grounding the winch just to the frame -- you want the connection to be as direct as possible, with as low a resistance as possible.

During a heavy pull, you'll take all of the juice that your alternator can deliver and then the battery will provide the balance. If you connect the stock harness and the winch harness to the same post, you'll be certain of not overloading the stock wiring and fuses. A double-terminal battery makes this easier...

Randii

Randy Burleson
4x4Wire Managing Editor Emeritus
Mongrel Isuzu Amigo