Once you have removed that rear panel various Econoseal connectors should be visible including a six way with matching plug providing individual feeds to each stop/tail light. The replacement LED stop/tail unit unless purchased at much higher price from MMC, will just have bare coloured wires and a decode for which is stop, tail and common ground connection.
Either way the three wires with captive pins from faulty lamp will need to be removed from plug, allowing removal of lamp itself, noting red/black l/h sidelight, green/purple stop and black common ground positions. These pins are barbed to secure within plug body and need to be teased out with 3.2mm blade after backing out the rubber waterproof seals. If you have bought the replacement from say car builder solutions you will need to crimp on fresh Econoseal pins after sliding on seals before inserting pins in correct order into the now vacant slots followed by seals. Various kits of these connectors are available on eBay and there are YouTube videos showing how to remove and re-insert pins.
