If you do not have one you need to get a volt meter. This will help answer a lot of your questions. With out your engine running measure the voltage across the positive terminal and negative terminals of the battery, it should be around six volts maybe 6.3 volts, Start the engine and with the transmission in neutral have a helper rev the engine up 15 to 1800 RPM (about equivalent to 35MPH driving). Now measure the volts the same way you did before, now it should read somewhere around 7.0 volts (this reading should be steady, not cycling up and down). Next you can use your volt meter to check your tail lights. Remove the red lens and remove a tail light by pushing in a little and twisting (wear a glove so you do not cut your fingers if the bulb breaks). Now turn on the questionable tail lights and use the red probe on your volt meter and touch the center of the light socket, it will probably be a dull silver. Then with the other probe touch the inside wall of the socket (DO NOT LET THE PROBES TOUCH EACH OTHER!). The volt reading should be around six volts engine running and five volts engine off. These volt readings are necessary if you want bright tail lights. This same procedure can be used on the head lights and volt reading are the same.