Mercury - MP Alert Y-Harness Expansion Splitter for Additional Lights 84-8M0053250. Overview. If you're expanding your MP Alert setup and want to add a third or fourth light, this harness makes the job painless. The Mercury 84-8M0053250 MP Alert Y-Harness creates a clean, plug-and-play split in the MP Alert wiring so you can connect extra oval smart lights without modifying the original harness. It's the simplest way to boost light coverage around the transom for better swimmer visibility and ov