Possibly the light is on the same 12 volt
circuit as the pump. Look for a loose connection somewhere on that circuit starting at the power panel and the fuse.
Or the light is responding to the vibration. Try banging gently on the light and associated wires with the eraser end of a pencil. Leave the pump turned off. The idea is not to beat the light and wires to death but vibrate it some to shake out the problem.
Rich, KE4GNK/AE, Overkill Engineering Dept.
'The Silver HamShack' ('07 International 22FB CCD 75th Anniversary)
Multiple Yaesu Ham Radios inside and many antennae sprouting from roof, ProPride hitch, Prodigy P2 controller.
2012 shortbed CrewMax 4x4 Toyota Tacoma TV with more antennae on it.