Try powering the electric lift with a pair of jumper cables directly from your battery (one 12V positive and one negative). If the lift works better then try just providing 12V power through jumper cables (this tests whether electric lift has good ground). Then try just providing ground through the jumper cables (this tests whether electric lift has good positive supply). Once you do the above tests, you should have a better idea whether the problem is the lift itself, positive wiring or ground circuit wiring (or all of the above).
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