OK photo time :
You wil need to put your negative probe from your meter on a 0v connection ie X1 the blue connection from the battery
1. First check you have 12V at R1 by placing your positive probe at point A on the photo.
2. Check you have 12v leaving R1 by placing your positive probe at point B on the photo - the other side of the resistor - if not you wil need to replace it.
3. If all ok with R1 check you have 12v going into the regulator by placing your positive probe on point D (ps I have removed the heat sink on this one for repair)
4. Now check the regulator is supplying the 5v you need by placing the probe on point C. You should read 5V if not the regulator will need changing.
Let us know how you get on