The MOT tester is required to test the operation of any side markers present:
The Tester will inspect:
Headlamps for aim and operation
Sidelights (known as position lamps) for location, operation and condition
Side marker lamps and end outline marker lamps (as appropriate) for presence, location, operation and condition.
Stop lamps for presence, operation and condition.
Daytime running lamps (for vehicles made after 1/3/1
Direction indicators and hazard warning lamps for presence, condition and operation and flashing frequency.
Front fog lamps (for vehicles made after 1/3/1
for presence, condition and operation.
Rear fog lamps
Reversing lamps (for vehicles made after 1/9/09)
Rear registration plate lamps
The cab radio isn't listed as an item to be tested so can be non functional. Any side markers that have been installed have to be operational.
Its as simple as that.