Tailgate release only working off key - not handle

[treeman]

My boot release now only works off the remote key fob, if I press the button under the tailgate handle it does not release any more. This started as an intermittent problem but now is all the time.
I took the lower tailgate lock mechanism apart and found the spring had broke but checked and BMW only sell the complete mechanism. I purchased some springs and replaced it and it worked ok for a month or so but it's not working again.
Having taken the mechanism apart, I'm wondering as the actual mechanism works fine from the key if it might be a fault in the handle release button on the upper tailgate handle part. I have disconnected it from the car and checked for resistance on the connector terminals when the button is pressed and there is no resistance, so my basic knowledge tells me there is no connection being made when the release button is pressed.
Is my thinking flawed? Is it more likely still the mechanism spring or is there some way I can check 100% if the release button is working?