[dsraven]

yup, sounds like a ground problem. to be sure, remove the bulbs front and rear, then turn on the park lights and see which wire is hot at each socket. be carefull not to ground the test light or a fuse will (should) blow. I usually get around this by simply putting a straw or piece of vacuum hose over the test light so only the end of the tester is visible. label those wires. then turn the park lights off and turn on the signals, key likely needs to be on. sometimes I simply turn on the 4 way flashers as they use the same wire and the key doesn't need to be on-especially if you are running the old points ignition system. test the sockets for that all around and label. it may not flash because there is no bulb for resistance but it should power on that circuit. label those wires. now jamb something on the brake pedal so the brake lights will activate and test the sockets for that. it should only light up the rear sockets and should be the same wire as the 4 ways and signals.
once you get the wires in the right places for each light, so the signals and brakes light up the bright element on the bulb, then ensure each socket also has a ground wire that goes to, usually, a body panel. then make sure that body panel is also grounded. sometimes the wire looks excellently connected to the body panel but the panel itself isn't connected to a ground because of fresh paint or whatever. for the front lights the ground may be to the rad support but then the rad support isn't grounded. I usually run a ground cable from the battery to the frame and another to the engine. then smaller wires to each body panel that is used for a ground and also to the cab. bare metal, star washer, some sort of anti corrosion over that when done.
if you run led bulbs your system may need some tweaking because they don't draw much so an electronic flasher unit may be required. also, they are polarity sensitive so they need to be wired correctly and will need a good ground and clean connections inside the bulb sockets etc.