With the other buttons "working" it sounds like it could simply be a case of bad/dirty contacts, but how one would get into it to check/clean I don't know.
If you do manage to get into it though, this
thread at hifi-remote
suggests it could be JP1 compliant, which opens up a whole new, wonderful world of programming capabilities, but there are others here far better qualified to discuss such issues.
Alternatively, genuine replacements
are available, but as with most OEM models, they are ridiculously expensive.