Radioshop

As previous posters have indicated, buy a voltmeter. If you have any money left over, get a test light too. The test light is easier to use when looking for a ignition circuit. If you penetrate the firewall, use a grommet.

Doing two-way radios for 15 years, I would not advise going under the hood for your battery connection. For what you need, whatever is in the fuse block is fine. Also NAPA sells these nifty little devices that plug into the fuse block that allow you to add a circut. Fuse Tap, Tap Circuit w/ Dual Fuse Holder . I routinely use these in heavy use vehicles with great success. Make sure you have a good ground. Try to piggyback on an existing ground somewhere. Look for a bolt down ground wire and add your ring to it.