Need some fuse box electrical help ideas 69 Mustang

[02Lightning]

When I built my car I added some gages and jumped off of the switched radio circuit assuming that a newer radio would draw less current than an old radio. Every now and then it will blow a fuse, so now I want to fix the problem. I was looking at the fuse box and I have an open accessory circuit (20 amp. Seat belt warning, swing tilt column & speed control). My car did not come from the factory with any of these accessories so one fuse holder clip is missing and the other has switched power. How can I jump off of this and run a wire to power my gages? I was thinking an Allen screw head about the same size as a fuse and maybe two jam nuts at the other end with a 20 amp inline fuse, and then run it to my gages. I know a lot of people have had to run into this, any suggestions?

[Sluggo]

I would definitely use that circuit. 
The only two safe options in my opinion are:

• use a t-tap and tap the wire from the back side and use and inline fuse (this method while not the preferred, will maintain the integrity of the harness)
• cut or strip the wire back and solder your wire and inline fuse in

[02Lightning]

Problem with that is I hate to mess with my under dash wiring.

[Midlife]

For 1969 and 1970, there's a slide-in fixture that adds the second fuse clip and an accessory 3 prong plug just for what you're talking about.  PM me and I'll see if I can cannibalize one for ya!