Thanks for your prompt and insightful response. I opened the motor cover and the armature or commutator looks good but dirty. Brushes appear to be worn. My guess is to clean the armature, but with what? Then I replace the brushes, reassemble and test.
The wiring on that looks a little funky. It has been modified where it goes into that block. Isolate and identify the path of the two wires and hook up a positive and negative bypassing the block. That black block is a insulator and not conductive. That ground wire connected to it is not going to work.
Try connecting a positive to that wire that goes up to the motor and ground to the frame somewhere. That green wire is not connected to anything.
You also have an issue with that commutator. It is very worn. Take a cirucuit testor and test the commutator like this. Set to resistance and touch two commutator plates. See what the reading is. Move one plate to the left or right, see the reading. Continue in that direction one plate at a time until you have gone all the way around. The readings should be the same or very close. If any two plates have a wide varying reading than the other, you have an open circuit and a bad armature. To make this make more sense, if you number the plates 1 through 4. Test 1 and 2, then 2 and 3, then 3 and 4 then 4 and 1.
My question is what did you initially hook power to? The 2 brass posts in the black insulator are NOT positive and negative. They are supposed to both be positive, one for the siren the other for the front light. Ground is through the siren body. So you would attach positive to the brass post with the black wire going to the siren motor and touch ground to any metal part of the housing.
Questions to ask.
1. Do the brushes move freely? (frozen brushes will not power the armature)
2. Is there continuity between the wire where it connects to power and the brush (could be broken wire)
3. Does continuity pass from one brush across to the other via the commutator?
4. Does continuity pass from the power wire to the negative wire? It has to pass from the wire, through the commutator and through the electromagnetic field and out.
4. Is the armature bad? have to test it as I stated in previous post
5. Is the ground corroded and not making good contact for the motor wire.
6. Is the ground between the motor housing and the siren housing corroded. (Use motor housing as grounding contact)
6. Does the fan spin freely by hand.
If all above is good the motor should spin. If it doesn't spin, one of the answers above will be no.