With that additional information, it confirms for me that the problem is between where the aftermarket system is spliced in to the lock wire and the lock relay. You need to keep tracing that wire until you find the break. If your test light has a good enough tip to pierce the insulation, that will speed things along. Just be aware that if the wire could be exposed to moisture where you pierce it, you will want to cover the small hole with either some tape or RTV, etc. to keep moisture from getting to the copper wire.

-Rod