Voltage to the Low Beams comes from the Step/Dipper Relay (terminal 56b, Blue wire), back into the cabin to the Bulb Failure Sensor, then out via separate Blue wires to the receptacles you metered.

Meter voltage at Treminal 56b on the Dip Relay. If it's still only 7.5v, it could be the relay, since it's input (Terminal 56, Yellow) is common to both High and Low beams.(Check for 12v on Terminal 56)

If voltage at Dip Relay is good, suspect the Bulb Failure Sensor -- maybe just working the connecter plug in and out afew times is all it needs. It is a round canister-shaped thing (Red ?) somewhere above the Driver's knees. Maybe clipped to the lower edge of the dash, out of sight.

Hope this helps,
--
Bruce Young,
940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified--and going to be crushed unless...)
New 144 from '67 to '78
Used '62 122 from '63 to '67