I repair and service DC machines for a living, so I might know a thing or two about brush/commutator maintenance ;-)

The brushes and commutator are porous. The thin oil in the WD-40 will soak in and "sweat" back out when the motor runs and heats up. Over time, it attracts dirt, dust, carbon dust, and moisture and creates an abrasive/corrosive layer that literally eats the copper off the commutator. If you've ever seen a deeply grooved commutator with a thick black layer on it, someone oiled it. The brush/comm interface should be nothing but copper/carbon. If kept clean, it'll last a very long time. If not, it won't. I should mention that oil is also flammable...

The motor will be switched either on the positive or negative side through both a switch and the motor regulator assembly (resistors in older cars, transistors in newer ones). One side of the motor will always be controlled in some form or fashion. The ONLY way for the motor to run when it shouldn't be is for there to be a short to ground in a system that's got the switching on the negative side of the motor, or a failed/stuck contact on one that is switched on the positive side.

The health of the motor has literally nothing to do with it running when it should be switched off. By fiddling with the wiring under the dash to drop the motor out, you may have "cleared" the short, or pulled hard enough on some wire somewhere to free up a stuck contact.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]