Very Strange electrical issue 98' F150
 

98' F150 4.6l V8
Truck will either start or have no power what so ever. Power locks wont even work.
Thursday
started fine to run errands, in the store for 45m came back out nothing. Disconnected battery cleaned terminals started right up. All electrical systems worked with key in accessory and on position it just wouldn't start.
Friday
Started fine to run errand. Started fine when leaving, I immediately shut back off because I forgot something. 10m later wouldn't start. Checked terminals they were a little loose gave them a wiggle started right up. Had one more stop to make and truck started fine. Didn't check whether different things worked at different key stage
Saturday
Wouldn't start. Will loose power than shortly after power lose will hear relays in fuse box start clicking like I am getting power. light will come back on and key ding will resume. Tried again same thing. After a few more attempts it started a cycle where it kept loosing power and regaining partial power all by itself. at least near as I could tell by the

It seems I completely loose power when I turn the key. When I bring the key back into off it usually comes back. I figure its either a bad relay, fuse, or short. One potential unknown, while checking under the dash I learned someone put an after market antitheft system in this POS. I thought those just prevented you from starting not killing all power all together but don't really know.

Help Please

Get your battery load tested. Clean all terminals and connections, not just at the battery.

FIRST, do as GLC said and get your battery tested. Check your battery cables and see if the wire inside of them has corroded. Check the wiring on the back of the fuse and relay panels. If you don't find anything, replace the battery, yes, even if it checks good. Johnson Controls has been flooding the market with ****ty batteries for close to 10 years. Go to where you bought your current battery and make them tell you who it is made by. Not who's name is on it but who actually made it. I'll give you three to one odds that it was JC. ALL the batteries that Pepboys, Autozone and Discrap Auto Parts sell are made by JC. Motorcraft batteries FROM FORD, I think are still good, but expensive. I've started using batteries from Walmart and they're holding up well and they have a four year FULL REPLACEMENT warranty.

Bought the truck used about a year and a half ago and its the battery that was in it. I've checked the ground and as many of the connections as I could before the Uber showed up to take me to work. I think its a connection issue somewhere near the power distribution box. I figured I might as well just start checking fuses. When I pulled out Fuse #28 (Trailer Electronic Brake Controller) my engine compartment light came back on and key left in ignition ding resumed. Attempting to start dash lit up in run but on start nothing happened and relays under dash began going off again. I'll get the battery checked out and check wiring in and around power distribution box.