The pyramid thingie on the dash has nothing to do with the central locking. It serves two distinctively different purposes. One, it houses a day/night indicator for the ECC system in the form of a light sensor behind the white opaque window, two it houses an alarm indicator in the form of a red led. If the car has neither ECC nor alarm it has no pyramid thingie, but remote central locking is completely independant of the alarm. My car has no alarm, but has ECC, therefore it has a pyramid thingie the red led part of which is not operational. My car also has remote central locking.

In order to check if the car has remote central locking, one can ask Yannis for help. I believe in North American market in '97 all 850s came with central locking (I may be wrong on this). It was certainly the case in '98 with S/V70s, one of which I own. If Yannis says it was an option, you can buy an appropriate key fob on ebay for not very much money and follow the procedures for programming it (look on Bay13 or ask here for those procedures). If you succeed, you have central locking, if not you have either not followed the procedure properly or you did not buy the correct key fob or you do not have central remote locking.

Hope that helps, good luck.
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Vladimir. 1998 S70. Base, 5-speed manual.