assuming, you did not make an election in the prior year to forgo filing form 1116. the FTC is carried back by filing an amended return. if you made the election the carryback is not allowed.

the carryback goes on line 10 on the 1116 for the same category of income as the carryback

if there is already a carryforward from that year the carryback will not benefit you. 

also, be aware that generally the FTC is limited to the US tax multiplied by the ratio of foreign taxable income of that category to total taxable income

@danieljameswatso  agreeing with @Mike9241 's great reply,  may I just point out that something @Mike9241  said  on the last line of his note ----  that  you can avail of the foreign tax credit if, and only if , there was foreign income  of that category -- this is implied  by the ratiometric ( foreign income  to world income, taxes thereof )  treatment of the available credit.  It may not be obvious till you compute the  allowable credit for the year.