ya its around 10-15bucks... BUT i use that light tester ALL THE TIME...... works on LOTS of stuff on your car.......
testing wires for power......... charging up amplifier... ive got several around the shop and one always in my car...

preaty versital........ the digital meters are beter for SOME things.... but doesnt compare to a cheap @ss light tester when it comes to your everyday mods......... only thing i use mine for is to check ohm load on my speakers/subs... thats it! and i spent like 150bucks for mine

 

i like my logics probe, plug it in cig lighter (lol so i guess that wouldnt help in this case, it also has an atachment with clamps for the battery though), poke into wire and it has a red light for positive and a green light for ground. but it set me back about 150 from matco

 

ya i got a good tester that has voltage ac and dc, current, ohms, and on top of the basics its got continuity and a diode tester. i needed a good one cuz i was a car audio installer and i paid like 50 bucks.

also did you check the fuse on the actually radar plug? there should be a cylinder fuse inside of the plug itself and the tip should screw off. make sure that is good before you tear your car apart.