On some cars, there is a terminal on the fire wall where the battery cable passes thru it. On others, you can attach to the terminal on the starter or the alternator. You can also use a set of jumper cables from the battery to the timing light. I like to install a small fuse panel similar to a MGB fuse box behind the passenger headlight. It has constant and switched power there. From that panel, I can draw my power for things like electric fans and relays, or use it to attach a timing light. Makes working on and modifying TR8s a little cleaner and easier.