Erratic auto antenna

[Useless angler]

Hi,
Has anyone experienced this and found a solution?
My auto antenna work just fine...apart from it goes up and down at will....it's will!....If no other electrics , such as lights, blower etc are on it generally stays up but starts misbehaving when I switch these thing on unless I am at high revs.
Have replaced the antenna, but not the motor.
Two new batteries fitted about 8 months ago
Tried a replacement relay...no difference.

I can see who this might be a voltage issue (given the issue gets worse when revs are low)

I want to keep the auto antenna facility

thanks
Paul