I might be wrong but it may be you are actually compensating for a different fault. The path taken, chasing the distance between the key & the sensor doesn't explain why it was working - then it wasnt. & now its iffy, If the distance was so crucial you would have experianced issues since day one.
These things( Key sensoring immobilisers) do have issues when the battery voltage is low. so i would start by checking the voltage at the battery , then the voltage at the sensor down stream of the soldered connection (Possible dry soldered joint).Im not familiar with your cars wiring , however if the voltage is ok , the next step would be to check the impedance to the Immobiliser Module.& so on through the circuit .
It may be there is a high resistance in the circuit which isnt apparent during the dry balmy days of summer - but come winter the.? This is exactly why people suddenly get battery issues in the winter .
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