Reversing Camera Monitor AV Input plugs

My Chinese GPS got knocked off by some low life whose need was greater than mine.
I purchased a replacement GPS with A/V input however the existing AV input plug is too big. The original cabling from the reversing camera is 3 wire cable terminated on a 3.5 mm plug (similar to external speaker plug ) with four segments on the plug
The new GPS has a 2.5 mm socket (I think). I bought a 3.5 mm to 2.5 mm adapter
but it did not work, On investigation it only had 3 segments on the male adapter plug
compared to four on the cabling plug.
Any ideas on how to solve this problem in our shrinking world ?