In order to prevent the splice from failing prematurely, you need to seal out the elements. The push in connectors do not provide any sealing or protection.

The best splice is to always strip the wires, slide the appropriate size heat-shrink tubing over the wire, wrap the wires together to provide a secure mechanical connection, solder the connection, slide down the heat-shrink & heat to seal the connection.

Butt splices when crimped with a real crimp tool and covered with heat shrink is a distant second choice.

The little blue in-line snap connectors are a sure failure.

Also, make sure you use star washers on both sides of the ground to chassis wire terminals - after cleaning any paint off the chassis at the connection point.