If you put a jumper from the battery plus to the post on the relay that the yellow wire connects to and then momentarily ground the the post on the relay that the brown wire goes to, the car should start cranking if the battery is sufficiently charged. Again remember it will start in any gear doing so.
If the starter engages and cranks, doing this, go back to where soundcontrol told you to check the yellow wire for 12 volts on the yellow wire when the key is held in the start position. If it does not have 12 volts, or close, then go under the dash and see if the yellow wire has power when in Start at that point. If nothing there, it would appear to be in the ignition switch as dodj said altho I would check the red wire carrying power to the switch as well.
If you have power in yellow with the key in Start under the dash, then the bulkhead connector is the likely culprit as dodj said.
If you had power at the relay from the yellow wire with the key in Start, then the relay is probably bad as soundcontrol said earlier.
I am assuming you have the wires connected to the relay as shown in the manual. If you are not sure, or need a picture, we can post one.