First thing to do is see if you have 24 volts AC coming out of the transformer. If you can trace the red and yellow wires coming out of the transformer to where you can put your meter on a connection. With power on you should see about 26 volts AC. According to the drawing the yellow wire is spliced to a red wire and a black wire. The black wire goes to the Low Speed relay. You might be able to read between the red wire on the terminal strip and the black wire on the relay to see if you have 24 volts AC there.
In looking at the photo; I don't see any wires coming off of the terminal strip with the G1 and G2 terminals. Only the wires that look like they go into the control box. Is there more that one wire connected to the terminal G1?
Where is the thermostat located? On the unit?
If you have the 24 volts, you could try and jumper between "R"terminal and "G2"terminal to see if the low speed fan relay will pick up.
I'll wait to hear from you before we move on.
Try to answer each of my questions as best you can.