Check that TL/CTSY fuse again - with a meter this time. Don't go by the link in the fuse window - sometimes they break way over to the side and you can't see it.
The good news is; if you have headlights, it's not the fusible link. There are two fusible links buried down by the starter. One is dedicated to the lighting circuits - the ones that are controlled by the HL switch. (The brake lights and directionals are entirely segregated from the other lighting circuits). I'll bet you don't have front parking lights either - the front directionals will work but not the steady parking lights. Your dome light and horn are probably out as well.
It goes like this:
The power to all the lighting circuits (except the directionals and brake lights) passes through the fusible link down by the starter. It runs up and across the firewall, the drops down to the firewall connection block.
It enters the firewall side of the block as a single big red lead. When it emerges on the cab side of the connection block, it immediately splits off into two separate (red) leads.
One of these leads provides power to the headlights only - and is run directly to the HL switch. This lead by passes the fuse block and is protected only by the fusible link. If you look at the switch harness connector, you will see it in come in at the top of the long row:
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But you have headlights, which means the lead dedicated to that circuit is hot. So, forget about that and see if you can find the problem with the other lighting circuits. All the other lighting is protected by fuses.
When the two leads are split after passing through the FW, one goes straight to the HL switch, the other goes to the fuse block. It enters the block at the line (inlet) side of the fuse socket labeled T/L CTSY (it is also shunted down to the line side of the HORN DOME fuse).
When it comes out of the load (outlet) side of the T/L CTSY fuse socket it is orange. That orange lead is then run to the HL switch where it provides switched power to the tail lights, front parking lights and dome/instrument lamps:
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NOTE: Don't go by the color of my tracers, I am way color deficient.
So you can see that if there is power to the HL's - but no power to the other circuits, there is a problem with either the splice at the FW (or the wire into the fuse block), the fuse block or the orange wire that feeds the HL switch.
If you have a meter and know how to use it, I would ring the circuit out like this:
1. Check the fuse for continuity.
2. Check for power on the line side of the fuse socket. Pull the fuse and check for 12 VDC coming in to the socket. It should be the clip on the RH side looking forward, but check both sides to be sure.
3. If you find that there is power into the fuse, check that it is getting over to the HL switch on the orange lead. Confirm 12 volts at the harness connector where the two orange leads enter the back. (that extra orange lead splits off to feed the courtesy lighting circuit).
If you have power coming into the HL switch on the orange, the tail lights should work. If not..IDK, recheck your connections.
