Start at the feed for the brakes and go through and clean, solder, grease and heat shrink every connection. Where any wires attach to ground, cover the ground screw/bolt with grease. This is especially important if your rig sees much rain while traveling and critical if ever in salt or other ice control products.
When you start cleaning the connections watch for corrosion which has traveled up the wire. Look for any nicks or damage where corrosion has traveled up the wire and created high resistance. There are various electrical greases available. I was just under my rig yesterday and noticed how damp and greasy looking my grounds are over two years later.