Why not just run the leads through the bulkhead behind the glove box, big rubber grommet there, connect direct to battery but charge from inside, if it's the lidl type one or similar, unscrew the croc clips & you've got a couple of nice ring connectors there to connect to the battery securely. That way you can charge from inside, just plug it in, select mode & off you go

You can't polish a turd - but you can roll it in glitter.