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Note: When replacing the struts, the mounts will also likely need to be replaced. Have these on hand to speed the repair process.
1. Raise vehicle, block front tires, support with jack stands.
2. Remove rear tire.
3. Install a 6"x3/8" bolt with a washer and nut on it through the hole on top of the auxillary damper (looks like a large rubber bumper behind the caliper). There are threads at the bottom of this opening.
4. Tighten the nut so that it compresses the auxilary damper slightly (just a few turns once it touches the damper).
5. Using a 15/16" wrench, loosen the nuts on the strut to knuckle bolts. These bolts are knurled so there is no need to hold the bolt head while loosening the nuts.
6. Loosen the two strut to body bolts at the top of the wheel well.
7. Lightly support the knuckle with a jack.
8. With the strut to knuckle bolt nuts loosened to the ends of the threads (to protect the threads from damage) knock the bolts through with a hammer. If it feels like there is still considerable tension on these bolts, it may be necessary to adjust the jack or the auxilary damper.
9. Remove the two strut to knuckle bolts and the strut to body bolts.
10. Remove the old strut and mount from the vehicle.
11. Prep the new strut and mount as per the manufacturer's instructions. This usually entails turning the compressed strut counterclockwise until it slowly decompresses. Then, using an Allen wrench and a wrench, the mount is attached to the strut piston.
12. Place the new strut and mount in the vehicle and reinstall strut to body bolts and strut to knuckle bolts. Tighten to manufacturer specs.
13. Remove the bolt from the auxillary damper, replace the wheel, and lower the vehicle.

Source http://wiki.answers.com/Q/How_do_you_install_a_rear_strut_on_a_1994_buick_regal_custom_3800