I need to install a Marley baseboard heater and double line break thermostat. The wiring diagram that came with the thermostat make no sense to me. It references line 1 and line 2. The wiring in the heater itself doesn't make sense to me either. Nothing indicates what is positive or negative. Do you have a diagram that will show the wiring from the power source to baseboard and thermostat whichever way it goes?

There is no positive and negative with household alternating current.

The 2 wire feed from the breaker/panel will connect to the Line on the thermostat. This leaves the other two wires or terminals on the stat, these should be labeled LOAD, these two wires connect to the two wires at the heater.

You are dealing with 240 volts, so you really need to be sure of what you are doing, be sure everything is done properly, as this voltage can do some serious damage or cause serious injuries.

This leaves the bare or green ground wire, which connects to the equipment ground bar in the panel, gets spliced through at each location and needs to connect to each green screw there may be at the thermostat and the heater.

You looking for positive and negative indicates that you are not familiar with this type of wiring, so be careful.