[TR] Radio Question

[Bill & AnnaBelle]

Hi folks,
	On an original 1970 TR6, Standard  Triumph AM radio ... 
positive  or negative ground??

There are  three ( 3)  wires  coming  out, two brown together  like 
household power cords, and  one (1)  black  ...  I am thinking  the 
two are speaker  wires, and the black power  using  a  case  ground.

Any help appreciated
-- 
Bill  Pugh
1957 TR3 TS16765L
aka
Casper

AnnaBelle Pugh
1970 TR6 CC59179L
aka
Rosey
Wallace, CA