no start due corroded battery harness,how to disconect it on the junction point on 2003 Mercedes-Benz E500

I have strong battery and can not start car .this problem occured a six months ago,mechanic have fixed it but ,it happened again.
please give me advice how to disconect junction point ,and not break it

Asked by for the 2003 Mercedes-Benz E500
spray it down with an antirust to help break down the corrosion.

conector have plastic body,hove to oppen it
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I have had the same problem with a key