I'm wondering how you checked the bulb?  Often the filaments break from vibration in such a way that they look fine... 

A trick I learned when I was in charge of a fleet of semi trucks was to hold the bulb in one hand and flick the glass with finger of other hand while observing the filaments.  You'll often see the filaments vibrate away, obviously broken.  But then it will look fine again.  You can even feel the difference in vibration of a bad bulb in the hand holding the bulb compared to a good bulb. 

Either that or test the bulb with power or replace it with known good one.

Next I'd run a temporary jumper wire to ground and see if you can touch it to light fixture (key on, taillight on, lens removed) and ground the sleeve the bulb fits into...  actually sometimes you can just push the bulb sideways gently with your finger and get it to work.

Ok, here's the weird thing. With my light probe I'm getting power to both the yellow and the blue, when I have the tail light plugged into the harness I also get power to the ground when probing it. I have now tried three tail lights, and two different bulbs. Have I ever mentioned I hate electrical?!

Thank you for those suggestions Mark, I'm mentally exhausted but will give your suggestions a try later, or tomorrow...

If I understand you correctly, test light is grounded and when you probe the taillight housing tester lights up?  That's a bad ground.  What's happening is the light fixture is trying to ground through the test light (or meter).  If you use a solid jumper wire to make a good ground, the taillight itself should work.

Thank you Mark, I will give that a try.

Can you feel the bulb pushing on the spring in the holder when you insert it? Also, is/are the two brass terminals in the holder dead central in their respective holes and in line with the two contact points with the bulb when it is inserted? On one of the indicators on my RD125, the brass terminal in the centre of the holder was ever so slightly bent, so wasn't making contact with the bottom of the bulb.

Does the brake light work ok (sorry if you've said and I've missed it!)?

Just hold the good bulb & probe the blue to one bulb terminal but have another probe from the black to the bulb body. Maybe probe both bulb terminals just to test both filaments. The yellow shouldn't be live until activating the brake but I guess ya activating it to get power. With an ohms meter probe the black to the frame or engine fin, anywhere grounded just to double check earth which should terminate at the plug coil mount under the tank.

Alan, the brake light is non op along with the stop light. Yes the spring does push back against me when inserting the bulb and pins are not bent (but I'm getting bent!). I'm officially frustrated, I tried Marks advice but obviously can't find a good ground anywhere and now the test light wont light at all, not even off the battery... I have a good noco battery charger that charges 6v batteries but for the life of me I can't find the darn thing.

Morely, thanks for the suggestion of holding the blue wire to the back of bulb contact and then ground as well, good suggestion for sure but now I'm not even sure my battery has any power since it wont even light my test light. I put a volt meter on it and it's only showing 4.xx volts.

Regardless of that, if you start the motor even though apparently the tail is battery powered, the motor will be trying to charge the battery so in a convoluted way it will liven the tail wire so you'll get bulb action. Just saying that seeing your charger has gone walk about. Idle might be a few revs too slow but fast idle should do it.