On my '89 B250,
Fuse #7 is for:
-Stop lamps(~ 5- 6 amps with 1157 incandescents).Brightness and Amp draw increase with higher voltage reaching light 1157 socket.
-Cigar lighter( I have no data on actual cigar lighter amp draw)
-Key and HeadLamp buzzer.(ripped this device out on day one of ownership, no data on amp draw)
-Aux AC and Htr.( no data)
So anytime the brakes are depressed, ` 5-6amps of load is presented to the circuit that also powers the cigar lighter causing more voltage drop and a lower than actual battery terminal voltage. How much voltage drop depends on teh length of the circuit, the wire gauge, the total amp load, and the number and conditions of the connections between the plug and the path to ground terminal back to battery. Lots of variables.
These potantial shared loads on the circuit is enough to depress the voltage reading significantly ay cigar plug, from actual battery voltage. These shared circuits for above list are just from the fuse block, and do not represent shared loads of other electrics feeding from the circuits between battery and fuse block iin glove box.
So the ciggy plug voltage on a Dodge can be much lower than actual battery termial voltage, and likely reads lower than actual on most makes and models when the shareds loads are also on the cigar plug fuse circuit.
Step on the brakes and I guess voltage drops 0.1 to 0.2v at the cigar plug, and the 5-6 amp load of the 1157's brighter filament, is unlikely to be overloading the alternator. The battery voltage likely remains steady when brakes are depressed. Door buzzer still work? bet the open door lowers the voltage reading taken at the cigar plug too.
It can be proven with a multimeter, if one cares to take readings and record results.
Bypassing the spring loaded crappy connections of a cigar plug voltmeter is better, but true battery terminal vlotage will be higher when the brakes are depressed
