I've been a co owner of a well known window tinting company in Kent for the last 15 years and due to the legislation passed on 1st January 2004 (section 32 of the road traffic act) you cannot apply a tinted window film to any of the 3 windows forward of the B post.
As a test we applied cling film to the windows of my previous 500 and this took it below the legal limit of 70% taking it down to 58% VLT (visable light transmission).
All cars from the 1990's onwards without exception are tinted to pretty much the legal limit give or take a couple of % so you cannot apply anything as this will make it illegal.
If you don't think your glass is tinted then drop the window slightly and put a white envelope behind the glass and take a look yourself.
OK this took quite a long time to check but I think I have the answer and Jax, it is not quite as you recall it.
To start with it is not s. 32 of the Road Traffic Act. There are four Road Traffic Acts (that I could find in the last 30 years anyway) - 1972, 1974, 1988 and 1991. Section 32 of each of them relates to something else.
What you are referring to is, I believe, a provision of
The Road Vehicles (Construction and Use) (Amendment) (No. 5) Regulations 2003 (the "Regs") which did indeed come into force on 1 January 2004. And these regulations (a so-called statutory instrument) do gain their statutory power from the Road Traffic Act 1988.
Paragraph 3 of the Regs amends regulation 32(11) of The Road Vehicles (Construction and Use) Regulations 1986 by the addition of this:
“(11A) Paragraphs (10) and (11) have effect in relation to any tint, film or other substance or material applied to a windscreen or window as they have effect in relation to the windscreen or window itself.”
The original paragraphs (10) and (11) of The Road Vehicles (Construction and Use) Regulations 1986 provided as follows:
(10) Save as provided in paragraph (11), the windscreens or other windows constructed in accordance with the foregoing provisions of this regulation of specified safety glass, specified safety glass (1980) or safety glazing and specified in column 3 of Table II in relation to a vehicle of a class specified in column 2 of that Table shall have a visual transmission for light of not less than the percentage specified in relation to those windows in column 4 when measured perpendicular to the surface in accordance with the procedure specified in a document specified in relation to those windows in column 5.
[There is then a table which I cannot paste here but it basically says 70% for present purposes - see
http://www.legislation.gov.uk/uksi/1986/1078/regulation/32/made]
(11) Paragraph (10) does not apply to—
(a) any part of any windscreen which is outside the vision reference zone;
(b) windows through which the driver when in the driver's seat is unable at any time to see any part of the road on which the vehicle is waiting or proceeding;
(c) windows in any motor ambulance which are not wholly or partly in front of or on either side of any part of the driver's seat; or
(d) windows in any bus, goods vehicle, locomotive, or motor tractor other than windows which—
(i) are wholly or partly in front of or on either side of any part of the driver's seat;
(ii)face the rear of the vehicle; or
(iii)form the whole or part of a door giving access to or from the exterior of the vehicle.
So I think in fact the law remains at the 70% limit for front windows and windscreen - all this change in the law did was to clarify (ouch) that a film or a tint was illegal as well as the glass itself being tinted.
PS What scares the crap out of me - especially when I am on my bicycle - is when I see mothers driving with babies in the front passenger seat with a sun shade covering the front passenger window. Truly terrifying.
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