How to validate payment method by specific voucher code

[Erick]

Dear Masters,

I have some payment methods installed and also have a voucher module where  users can get certain discount by providing a valid voucher code.

The problem is how to link between these two modules ? I want specific payment methods shown based on voucher code.

For example, if i filled the voucher code with "PayCash" then only methods with transfer payment will be shown, or when i fill in  "InstallmentDay", then only installment method will be shown.

Which file page and which function that i should code or modify ? Or is there any add-on that I can use ?