How to connect an adapter to a power thief?

Hello, I would like to supply my amplifier separately with power. For this I would like to plug the harness of the amplifier directly to a fuse box with the help of a power thief. Now I have problems with the integration of the wire harness and the power thief. In the appendix you see the problem. The wire harness has a kind of flat pin plug. the power thief has a kind of Cinch port. I don’t like to work on the flat pin plug because I use the cable harness as original as possible. My idea would be to cut off the connection of the current thief and solder the bare wire onto the flat pin. However, I have concerns about durability and safety. I can’t get a shrink hose over there. I don’t think it’s very good with insulation tape either. Do you have a better, clean solution to the problem?