Not sure if it helps, but on mine which is a fair bit older, 96 FG637, when I got it the exhaust brake did not work, but light came on etc when you moved the lever. Turned out the brake was seized closed, not so good for the performance, or what I thought was an exhaust leak, but turned out to be an overheated cracked exhaust manifold, so one new exhaust manifold and exhaust brake fitted and all good.
Occasionally it still doesn't work and had me scratching my head, but turned out to be simple, the spring on the accelerator pedal was not allowing the pedal to return fully to it's non pressed state, preventing the micro-switch from activating it, just needed to give the pedal a pull up with my foot and the brake would work again
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