before you do anything check the voltage with the same meter to the battery and see if you get 6 volts from there.. i had a problem with just getting 8 volts from my meter.. it was broke.... fixed it and now i get 12 volts.. make sure the probes are clean so you get full voltage to the meter..... you suppose to get 12 volts to the lights and it is either the battery or the connections, which might have some resistance to them due to corrosion... clean off all the contact points and see if this helps.