Sounds like you've got a short in the kickdown switch or cable.

I know from banging my head against a wall for some time that the kickdown switch is on the same fuse as the blower motor.
Since it only happens when you kick it down, I'd guess you just need to replace the switch itself, which is cheap enough. But it's easy enough (if oyu don't mind reaching under there) to test for resistance on the wiring to the switch. I disconnected mine a while back, and use my hand to manually downshift with the automatic trans selector.

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