I doubt the card will tell you anything about 'constant" power. I had the same trouble with the clock and radio only working when the car was going (W123) so I rigged a "hot" jumper into the line that feeds the clock. Now all is well again.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"