prog report would be great.
I too have a rear light cluster access panel on the meg 3 both sides,but for access to fuel flap area ,not a prayer.
Whole rear panel removal doesnt look too bad(untried).
My last car ,mentioned above,the locking fuel flap aspect went after about 5 years,I just removed the solenoid and left it as a push to close flap..
I didnt bother with a locking fuel cap,under the flap.
Since you already have the replacement ,may as well try and fit it.
There are similar/somewhat similar threads for meg II on site.
The light thing: mine,foglight bulb replacement is supposed to be a dealer only job, but you can access them either thru the wheelarch liners or by removung the front plastic undertray.
You were probably looking for a similar "tip" on the fuel flap.
I unfortunately have no such info,and the modus isnt one of the larger sellers