Solving intermittent problems like this may take a while, so you're going to need to be patient as we step through this.
All wires from Fuse 21 are coloured Red (Rot) with a White (Weiss) stripe (Rt/Ws)
Here's a description of the circuit.
Fuse 21 >-rt/ws-> Join X207
X207 is in the Front Fuse Box.
Join X207 >-rt/ws-> Connector X15 Pin 22 >-rt/ws-> Charging Socket
Connector X15 is 30 pin, White and is located
"Below LH side of dash, near kick panel, left of the steering column"
Join X207 >-rt/ws-> Connector X14 Pin 16 >-rt/ws-> Join X435
Connector X14 is 30 pin, Yellow and is located
"Below LH side of dash, below driver footrest"
X435 looks like it is located beneath the rear seat in front of the rear fuse box.
Join X435 >-rt/ws-> Alpine Stereo Anti-Theft Module Pin 11
Anti-theft module is located "below LH rear seat"
Join X435 >-rt/ws-> Trunk Lid Switch
Trunk Lid switch is located "in centre of trunk lid in trunk latch assembly"
Join X435 >-rt/ws-> Connector X355 Pin 2 >-rt/ws-> Interior Rear Right Light
Connector X355 is 3 pin, White and in the RH C-Pillar
Join X435 >-rt/ws-> Connector X356 Pin 2 >-rt/ws-> Interior Rear Left Light
Connector X356 is 3 pin, White and in the LH C-Pillar
Connector X356 Pin 2 >-rt/ws-> Connector X336 Pin 2 >-rt/ws-> Front Dome Light
Connector X336 is 4 pin, White and located "In centre of windshield header" (up in the roof behind the panel that hides the sun roof motor)
I'm not sure what will happen if you disconnect the anti-theft module of the Alpine Stereo, so for the moment, I wouldn't that.
Try disconnecting each of the various interior lights, one at a time and see if the problem goes away. Sometimes when a globe fails, a broken piece of the element shorts across the 2 pieces of metal that normally support the element. Do you have an interior light that doesn't work ? This may be the problem.
Similarly, has the trunk light failed / try disconnecting the trunk light ?
I my "5", the connector for the charging socket is poorly protected (a piece of low density sponge). Has a bobby pin / paper clip or some other metallic foreign object fallen into this connector ? Try disconnecting it and see if the problem goes away. I think the Red/White cable plugs in to the back of charging connector.
Let me know how you go with all that and I'll see if I can think of a few more things to try in the mean time.
If none of that solves the problem, there's a distinct possibility that one of the wires in this circuit has compromised insultation. To blow a 10A on 12V circuit, the resistance from the appliance side (not the power side) of the fuse is going to be less than 1 ohm.
Do you have a multimeter to be able test this ?