I bought one of these awhile back and I will never install lights without it. Troubleshooting is especially easy. The last install I had the department had an issue with a horn on one truck and a light on another, in less than 5 minutes I had both problems diagnosed and told them how to fix them. Without it I would have spent a good 15-20 minutes using different testers and wire leads and such to identify the problem Using a multimeter compared to this is like using SLK lights to Whelen, there is just no comparison.
http://www.amazon.com/Power-Probe-Ultimate-Automotive-Electrical/dp/B000KPU8MK/ref=sr_1_1?ie=UTF8&qid=1291712788&sr=8-1
http://www.amazon.com/Power-Probe-Ultimate-Automotive-Electrical/dp/B000KPU8MK/ref=sr_1_1?ie=UTF8&qid=1291712788&sr=8-1