The courtesy (under-wing) lighting and the rear seat overhead map light were on the same switch. I didn't like having the bright outside light on while my passenger read his magazine, so I separated the lights to two different switches. What will happen when this is discovered during my next annual? And can a certified mechanic ground my airplane for it?
The short answer is that a mechanic can't ground a plane. There are lots of discussions online about this, and the typical scenario seems to be that a mechanic "refuses to sign off an annual" or something similar. (There are other scenarios too, where the mechanic refuses to return the aircraft and/or logbooks to the owner because of a dispute over payment or work performed, but that's a legal issue, not an aviation one.)
Who can ground an aircraft?
FAR 91.7 forbids a PIC from flying an aircraft that is unairworthy, although FAR 21.197 (Special Flight Permits) allows the FAA to grant special dispensation to fly an unairworthy aircraft (usually for repositioning purposes). There is no FAR that empowers a mechanic to ground an aircraft. Mechanics are not the safety police.
When it comes to signing off on maintenance, there are two cases: required inspections (e.g. annuals), and general maintenance (e.g. an oil change). Required inspections are governed by 14 CFR 43.11.
[...] if the aircraft is found to be airworthy and approved for return to service, the following or a similarly worded statement—“I certify that this aircraft has been inspected in accordance with (insert type) inspection and was determined to be in airworthy condition.”
[...] if the aircraft is not approved for return to service because of needed maintenance, noncompliance with applicable specifications, airworthiness directives, or other approved data, the following or a similarly worded statement—“I certify that this aircraft has been inspected in accordance with (insert type) inspection and a list of discrepancies and unairworthy items dated (date) has been provided for the aircraft owner or operator.”
In other words, even if the mechanic finds that the aircraft isn't airworthy, they're still required to make and sign a logbook entry to document that. The key point that Mike Busch makes is that what happens next is up to the owner, not the mechanic:
The owner must correct those discrepancies before the aircraft may flown. He is free to have them corrected by any mechanic he chooses. There’s no need for the inspecting mechanic to look at the aircraft again. The next annual comes due in 12 calendar months.
The owner might ask the same mechanic who did the inspection to make repairs, or a different mechanic at the same airport. Or they might ask the FAA for a ferry permit to move the aircraft to another location for maintenance. Or they might just fly the aircraft anyway, knowing it isn't airworthy.
As for other maintenance that isn't a required inspection, 43.9(a)(4) says (emphasis mine):
If the work performed on the aircraft, airframe, aircraft engine, propeller, appliance, or component part has been performed satisfactorily, the signature, certificate number, and kind of certificate held by the person approving the work. The signature constitutes the approval for return to service only for the work performed.
Here's Mike Busch's commentary on that:
Those two sentences bear careful reading. The first sentence makes it explicitly clear that a mechanic’s signature on a 43.9 logbook entry does not signify anything about the airworthiness of the aircraft. It signifies only that the work the mechanic completed was performed satisfactorily. [...]
The second sentence makes it clear that the mechanic’s signature does not constitute an approval for return to service for the entire aircraft (the way an annual inspection signoff does). It constitutes approval only for the work the mechanic performed.
So coming back to your specific wiring scenario, if the mechanic performs a required inspection and determines that the change makes the aircraft unairworthy, they would document it as a discrepancy per 43.11. That means the aircraft is not airworthy, but how to make it airworthy is your decision, not the mechanic's decision.
Finally, if you're not sure how a mechanic would react to your wiring change you should probably find out. It's possible that the aircraft is already unairworthy, but that depends on a lot of details: type of aircraft, certified vs experimental, any applicable ADs etc.