I'd run the output of the CTek to at least one of the three BeCM terminals under the seat if you want to connect it inside the car. If its an MXS5.0 then a 4mm cable from under the seat to the loadspace would be fine, fused down at the terminal to 10 amps. Yes 4mm is overkill for 5 amps, but you want a decent connection to minimise voltage drop between the charger's sense circuitry and the battery you're charging.
Otherwise (for example, using the tow bar perm 12v) you have a myriad of wiring and other connections that will add to the resistance and potentially confuse the charger. The wiring is all old now as are the connections. Those three BeCM feeds are 6mm^2 back to the fusebox, via a maxi fuse each and then onto the battery. About the best you'll find inside the car for charging.
A direct connection back to the battery (suitably fused at battery end of course) would be best. Or just fit one of the CTek weatherproof sockets onto the battery terminals and place it somewhere convenient?