Well... What I think is; used JDMs / CBUs / Imported Cars (New / Used) are not exempted from With holding Tax in any way. Just ye farak hai k locally assembled vehicles k case mein aapko With holding Tax directly assembler ko daina parta hai, however, in case of CBUs, woi tax aapko directly E&T department ko daina parta hai, at the time of registration of vehicle.
I still remember, when I had bought FAW V2 last year, I did not pay any With holding Tax to Al Haj FAW Islamabad Motors, since it was a CBU, however at the time of registration, I paid Rs. 25,000/= to E&T Islamabad directly. Now FAW directly charges Rs. 25,000/= as With holding Tax, since local production has started and FAW V2 is not a CBU any more.
So, tax ki ma'afi kahein b nai hai, just tareeqa change ho jata hai.
One is bound to pay it, one way or the other.