Disabling Form Text Boxes

I have seen it before on website forms however now i am wanting the same functionality.
Basically i have a form and i have a series of radio buttons called existing_locations and a text box called new location.

i want to two master radio buttons 1 called an_existing_location and the other called a_new_location

when existing location is selected then the sub set of existing_location radio buttons become active thus clickable however the new_location text box is disabled (greyed out)

if the new_location radio button is selected then the new_location text box becomes active (not greyed out) and the series of existing_location radio buttons are disabled (greyed out)

if anyone nows where i can find a script to achieve the above i would be very greatful

regards claude

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