A test lamp is a simple but very useful tool that you can build at home in a few minutes and it will help you solve a number of electrical problems in your car.
We need the following materials:
1 5W 12V lamp or similar, also works up to 22W
1 bayonet nozzle with cables
1/2 meter of black cable
1/2 meter of red wire with test lead (from a multimeter)
1 medium black lizard clip
10 cm of heat shrink tubing or electrical tape
Tools: Pliers, cutter, lighter
Put the alligator clip on the black wire and then join it to the black wire of the bayonet type socket, the red wire with test lead (it can also be a rigid wire if you don't have the multimeter lead) is attached to the red wire.
Isolate everything correctly and voila!!! Now you can do tests in your car.