The advantage with crimps is you don't get the stress point where the solder ends - the wire will be stiff at the joint, then flex where the solder hasn't run. This is a weak spot. However, if all the joints are sheathed in heatshrink, and are pushed into the body/tailgate they should be fine. Silicone sheathed flexible wire is available from R/C model suppliers. Giantshark.co.uk sell it, I would think that 18swg would be adequate. Mine hasn't failed yet after 3 years plus, but I'm standing by......