High/low mode selection is accomplished in the seat heater controller.
Same fuse runs the high or low temperature mode.

The relay looks to be set up such that in low mode, the back pad and keister pad are connected in series. In high mode, they are connected in parallel.

If you are getting heat out of both pads then chances are the problem might be in the relay or the switch since if both pads are making heat then they will be OK in high mode too.

When you choose high heat, does the "high" side switch LED light up? The relay tells that LED to light up to confirm the heat mode has been selected.

I wonder if there is an undervoltage protection mode similar to the rear defroster, that cuts out the heater power if the system voltage drops too low. You might be ok on low but the added demand on high might be tripping the relay protection.

On the defroster the LED will blink if this occurs. I suspect the seat heat system is similar.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech