Quick question on headlights.

I did not have the problem described until I fitted the rear rubber cover that was missing, I can only assume the rear cover is holding the moist air in, as removal of said cover cured the issue over a week or so.
Not the solution I know, but the headlamps are in a fairly well protected area anyway without need for the covers, in my opinion.
And yes, they are bloomin expensive.

Thanks for that! 
It does look like condensation to a point and I was hoping my DA polisher would warm the glass enough to disperse it.
I'm about to go and remove the rubber covers and give it a try.

Edit: covers removed and fingers crossed.

I did also clean the lamp glasses externally, I used very fine wet and dry paper, 1200 or some such and a proprietary 3M lens cleaning kit. It worked very well all in all.
I think I may refit the dust caps but punch a hole somewhere to allow airflow.

In my spare time I sometimes refinish guitars for friends and so I have grades from 80 -5000 and I usually start with about 1000 and work up to 3000. 
After that my DA will polish it to a mirror shine.
I did think about putting holes in the rubber but I will wait to see if removing them cures the problem.
Thanks for your help.

It isn't hard to remove the clear "lens" from the black headlight moulding, you just put it into an oven with the temperature set at 135C for seven minutes. Then using those plastic trim removal tools gently work your way around levering the lens off. You will find that you will have to reheat the headlight again before it eventually come off. To refit the lens you will need some butyl tape which you cut to length and fit into the black headlight moulding, another seven minute in the oven and press the clear lens into the butyl strip. It's not too bad after you have d0ne the first ne..
4PCS CAR TRIM REMOVAL TOOL KIT SET PANEL DOOR PLASTIC PRY DASH INTERIOR CLIP Completed

Butyl Tape Rubber Glue Headlight Sealant Retrofit Reseal Headlamps Door 4M 13FT Buy It Now

Wow MGB821....you have certainly done your homework and got stuck in to all things MGF TF!  
You make it sound so easy like baking a pie. Thanks for the info because if it doesn't clear on it's own I just might be having a go at that. I understand you have to remove the front bumper to remove the headlight unit? 
I need to sell my MX5 so I can get my TF in the garage and lay it up whilst I do a few jobs on it. The kind of jobs that don't have to be completed in one day and the car can be left in bits.
I see you put Bilsteins on your car..B8's by chance? Did you use std springs or uprated? I have them on my MX5 and it's quite a firm ride with upgraded springs. 
Thanks for any help on the suspension.

Yes, MGB821, a very good reply.
Is this already in the How To? If not, I think it should be.

If it's condensation, try removing the bulbs, and as I did, use the blower function of a workshop vaccum cleaner to blow through one of the orifices. ( Presuming you can blow clean air!) The air from the blower is usually warmer, so it'll clear the moisture after a while.

generous_dad wrote:

Yes, MGB821, a very good reply.
Is this already in the How To? If not, I think it should be.

I didn't look but it is already in the "how to's"  https://www.the-t-bar.com/forum/54-how-tos/96076-daytime-running-lights-cob-led-halo-s-for-the-tf#184927
I just typed from memory from when I fitted Halo lights to our TF
 

Pharg55 wrote:

If it's condensation, try removing the bulbs, and as I did, use the blower function of a workshop vaccum cleaner to blow through one of the orifices. ( Presuming you can blow clean air!) The air from the blower is usually warmer, so it'll clear the moisture after a while.

I'm not 100% convinced it is condensation. It's not like fine water bubbles it's milky grey mainly at the top edge. I have power next to where it is parked so I will try a hairdryer on it tomorrow and see how I get on. 
 

Managed to blow up a pic of when it was in the garage. I can assure you the outer lens has been finely polished. The marks you see are inside the lens.   

Last year i used a variation of Pharg55's method, removed the bulbs and left my daughter's hairdrier pointed into each headlight until they cleared. The fog is back again now so i will experiment with leaving out the rubber bungs to see if they self cure.