crimpcon red 50-pack

Crimp connectors are a great way to quickly make a waterproof in-line wire connection.
 
The "red" works for the most common sizes, AWG 22-16.
 
Strip wires about 1/4", insert into connector ensuring that stripped wires are inside the metal tube, crimp the metal tube on both sides (for each wire) and use a lighter or heat-gun to heat-shrink the plastic coating.
 
NOTE: the connector will be waterproof if there is ONLY ONE wire per side. 
Multiple wires per side can be used for indoor applications.