There was a factory accessory rear sun shade. I have no idea if it still available or not. This is the part number.
Manual Rear Window Shade
82249402930
I compiled a list of all the ti specific parts from a catalog that I got in Germany about 8 years ago. It's pretty comprehensive but I don't know how much of this stuff is NLA or unobtainable here.
http://www.318ti.org/forum/downloads.php?do=file&id=16 EDIT: Pelican Parts has it.
82249402930-BOE
Genuine BMW Part: SUN BLIND
- This is a special-order, non-returnable, BMW part
(availability and shipping charges may vary).
$154.25