Lights do not dim when I turn the key. What could be my problem or where should I start?

First you have to charge the battery and have it load tested. But, there is a 40 amp fuse int heunderhood electrical center. Also before you start changing switches check them first. You do have a neutral safety switch which the starter goes through as well. You can check the yellow wire with a test light to see if the ignition switch is working then the purple wire to see if it's going through the switch that would check the ignition switch and neutral switch. If everything there is ok it then goes to the relay which that could be a bad connection there, like the cable may have slipped out.