Scrasauge,
If you're lucky, it's just a faulty bulb and/or poor ground. Disconnect your battery! Replace ALL 4 -1157 bulbs (avoiding Sylvania, if possible) and while each bulb is out, clean, with a small brass brush, all contacts as well as the inside of the barrel of the socket, which provide the ground to the bulb. Lube the sockets with dielectric grease and remove, clean and replace the ground connections to the body. Over the years, I have had several 1157 bulbs develop tiny, hairline cracks in the base/insulator that allows a "slight" short from one circuit to the other, causing your exact symptoms. Please post back to let us know how you make out, as many folks here ask and get good advice to issues but never post back to confirm or debunk the advice. Oh yeah, like I said, "DISCONNECT YOUR BATTERY!"
HTH,
Gene