Mondeo 130TDCi Battery SmartCharger Issue

Hi My Ghia X TCi 130 Est 54 plate has a battery light lit 30sec after start that doesn't go off. Found a 7.5A battery fuse blown which blew again 30sec
after start up. Ford suggested a faulty ECU or smartcharge wiring fault related to the ECU or wiring which would cost mega . The alternator & battery are good when tested. The 7.5A fuse blows whether the smart charge is connected to the alternator or not & I don't have a scope to check the signals coming to alternator via the smart charge wiring but assume to be OK. Possibilty that heated front screen isn't working but fuse & all others seem OK & glove box fuses all seem OK. What can cause this fuse to blow
Any help or suggestions appreciated.
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