why or how can u have power on one side of fuse and not on the other 1969xke series 2 I have power on one side of fuse 3 and not on the other
The correct method of testing a fuse in a fuse panel is to test for power on both sides of the fuse while it is in place just as you have done.
Too many mechanics just remove the fuse and look at it to see if it is burned. But on any fuse especially on Lucas fuses and fuse boxes, the loss of connection can be the box itself, as they are noted for loosing connection at the clips that hold the fuse in. Especially on a 49 year old car.
So your first step is to remove the fuse and use Emory colth or sand paper to clean the insides of the clips in the fuse panel and sand the fuse itself and if it looks good either test it with an ohm meter or just put it back in the fuse box and retest it. Some fuses can look good and not burned but have no connection inside.