Electric aerials have a toothed-belt attached to the extending rod - you can often just get the new rod with belt attached rather than having to replace the entire unit.
I've replaced one myself in the past - it's just a case of turning the radio on and waiting a few seconds for the 'aerial' to fully deploy, removing what's left (if anything) and then feeding the new belt and aerial into the unit as you turn the radio off to retract it. You sometimes need to repeat this a couple of times for it to fully retract.
It's a lot cheaper and more covenient than buying and fitting a replacement unit.
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