Modern planters have at least two wiring harnesses that need to be connected. Some of the big, fancy planters have up to half a dozen wiring harnesses. Here are a few suggestions what to do if electricity isn't flowing between tractor and planter like it's supposed to:
-many planters get their main power supply via the 7-pin connector on the back of the tractor. There are sometimes other power sources---auxiliary power connections in the cab, or accessory power connections between the tractor and planter---but the 7-pin system supplies power to a lot of circuits on planters. When in doubt. use a voltmeter or test light to check for power at the tractor's 7-pin connector. The longest pin, the top pin, is supposed to be the ground. The center pin should be 12 volts with the ignition switch "on." Pins around the perimeter at 4 and 7 o'clock are often turn signals.
-if power is missing at the tractor's 7-pin connector, first check for a blown fuse in the tractor's main fuse block. If the fuse is okay, be suspicious of the backside of the actual 7-pin connector. For some reason there's often a lot of corrosion where those wires attach to their sockets on the back of the 7-pin outlet.
-if the problem is with a seed monitor wiring harness connector, or a can-bus wiring harness connector, double-check to make sure the pins in the connectors are mating correctly. It's easy to force those stubborn plastic connectors into place and accidentally bend, or more commonly, push one or more of the little male pins back into their sockets. Be aware that those male pins will pop back into place when you disconnect the main connector, and appear to be "okay." Push a small screwdriver against the tip of each male pin to see if they're solidly locked in place.
-whenever possible, apply di-electric grease to the pins and sockets in electrical connectors before you couple them. Di-electric grease isn't anything magical and it's admittedly a little messy, but it's a lubricant that makes connectors water resistant enough to significantly reduce corrosion.